I wouldn't worry about the humidity if I were you. Believe me at -4F the air will not hold much moisture - BRRR!
Remember that relative humidity (usually what they report on the news) is what the amount of moisture in the air relative to the total moisture it could possibly hold. The colder air gets, the less total moisture it can hold.
If you are really a nerd like me, you can check out this website that has a nice explanation:
Humidity But I *do* hope your chorizo turns out well! YOu are making my mouth water. My mom made some when we lived in Virginia (where it is humid and not very cold)- I will try to ask and see how she cured it.